Once in a blue moon running some of those scripts seem to change the actual background, but most of the time the values in the registry change but the image stays the same. As for when it changes, there does not seem to be any rhyme or reason behind it. I think I am missing something important about how Windows handles its background image vs themes.. or something?

I am trying to do all of it directly in Powershell (no C code, no extra plug-ins or extensions, trying to maintain a clean system)